Buses constitute a significant source of air pollutant emissions in cities. In this study, we present real-world NO_x emissions from 97 diesel-hybrid buses measured using on-board diagnostic systems over 44 months and 6.35 million km in London. Each bus had previously been retrofitted with a selective catalytic reduction (SCR) aftertreatment system to reduce emissions of nitrogen oxides (NO_x). On average, parallel hybrid (PH) and series hybrid (SH) buses emitted 3.80 g of NO_x/km [standard deviation (SD) of 1.02] and 2.37 g of NO_x/km (SD of 0.51), respectively. The SCR systems reduced engine-out emissions by 79.8% (SD of 5.0) and 87.2% (SD of 2.9) for the PHs and SHs, respectively. Lower ambient temperatures (0–10 °C) increased NO_x emissions of the PHs by 24.2% but decreased NO_x emissions of the SHs by 27.9% compared to values found at more moderate temperatures (10–20 °C). To improve emissions inventories, we provide new distance-based NO_x emissions factors for different ranges of ambient temperature. During the COVID-19 pandemic, the emissions benefits of reduced congestion were largely offset by more frequent route layovers leading to lower SCR temperatures and effectiveness. This study shows that continuous in-service measurements enable quantification of real-world vehicle emissions over a wide range of operations that complements conventional testing approaches.

中文翻译：

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伦敦公共汽车的长期使用中氮氧化物排放与改造氮氧化物后处理

公共汽车是城市空气污染物排放的重要来源。在这项研究中，我们展示了伦敦 44 个月和 635 万公里使用车载诊断系统测量的 97 辆柴油混合动力巴士的实际 NOx_排放量。每辆公共汽车之前都安装了选择性催化还原 (SCR) 后处理系统，以减少氮氧化物 (NO _x ) 的排放。平均而言，并联混合动力 (PH) 和串联混合动力 (SH) 公交车排放量分别为 3.80 g NO _x /km [标准偏差 (SD) 为 1.02] 和 2.37 g NO _x /km (SD 为 0.51)。SCR 系统将 PH 和 SH 的发动机排放量分别减少了 79.8%（SD 为 5.0）和 87.2%（SD 为 2.9）。较低的环境温度 (0–10 °C) 会增加 NO_x PHs 的排放量减少了 24.2%，但与更温和的温度（10-20°C）下的值相比，SHs 的 NOx 排放量减少了 27.9%_。为了改善排放清单，我们针对不同的环境温度范围_{提供了新的基于距离的 NO x排放因子。}在 COVID-19 大流行期间，减少拥堵带来的排放优势在很大程度上被更频繁的路线停留所抵消，从而导致 SCR 温度和效率降低。这项研究表明，连续的在役测量能够在广泛的操作中量化现实世界的车辆排放，补充传统的测试方法。